1. Field of the Invention
The present invention relates to an optical element and an optical device incorporating it and, more particularly, to those suitably applicable to video cameras, still video cameras, copiers, etc. making use of the optical element with a plurality of reflective surfaces having curvature.
2. Related Background Art
CCD image pickup elements used in the video cameras, digital cameras, etc. have their sensitivity in a relatively wide wavelength region and also respond to light other than the light in the visible region. Since color filters normally used for the image pickup elements transmit the light of wavelengths except for the visible rays, this would become a false color signal to pose a trouble in color reproduction. It is thus common practice to interpose a filter for removing the light of unwanted wavelengths in an optical system of the presently available video cameras, digital cameras, and so on. Colored glass sheets, for example, equivalents to color glass CM-500 (available from HOYA K.K. etc.), have been used heretofore as infrared cut filters commonly used for removal of infrared rays. It is also necessary to use a filter for removing ultraviolet rays when a sensor used is sensitive to shorter wavelengths.
There are three probable approaches as methods for cutting the light of unwanted wavelengths.
(a-1) The light of unwanted wavelengths is absorbed, while the light of necessary wavelengths is transmitted.
(a-2) The light of unwanted wavelengths is transmitted, while the light of necessary wavelengths is reflected.
(a-3) The light of unwanted wavelengths is reflected, while the light of necessary wavelengths is transmitted.
Materials used in the method of (a-1) include resin compositions invented as described in Japanese Patent Application Laid-Open Nos. 6-73197 (corresponding to U.S. Pat. No. 5,514,740), 6-222211, etc., in addition to the colored glasses. An optical element used in the method of (a-2) is a cold mirror. Materials used in the method of (a-3) include dielectric multilayer films for preventing transmission of infrared light, and the like.
On the other hand, it has been clarified as to non-coaxial optical systems that an optical system well corrected for aberration was able to be constructed by introducing the concept of a reference axis and making component surfaces of asymmetric, aspherical surfaces; for example, Japanese Patent Application Laid-Open No. 9-5650 describes a design method thereof and Japanese Patent Application Laid-Open Nos. 8-292371 and 8-292372 (both corresponding to U.S. Pat. No. 5,825,560) describe design examples thereof.
Such non-coaxial optical systems are called of-faxial optical systems (which are optical systems defined as optical systems including a curved surface (off-axial curved surface) a normal to which at an intersecting point of the component surface with the reference axis does not lie on the reference axis, where the reference axis is considered to be along a ray passing the center of the image and the center of the pupil, the reference axis being of a bent shape). In these off-axial optical systems, the component surfaces are normally non-coaxial and there occurs no eclipse even at a reflective surface. It is thus easy to construct an optical system of reflective surfaces. They also have such features that routing of the optical path is relatively free and that it is easy to construct an integral optical system by integral molding of component surfaces.
Since the conventional methods for removing the light of unwanted wavelengths have employed the single-unit wavelength selecting filter interposed in the optical system, they had the problem that an extra housing space was necessary for the filter and it impeded downsizing of the lens system. The back focus must be longer by the thickness of the filter and this posed the problem of restrictions on the design. In addition, the above structure necessitated a member for holding the filter.